TESTING WITH ULTRA VIOLET (UV) LIGHT

To buy UV lights: SHOP NOW

I have divided this article into three sections, you don't have to read all of it!

Plain-English summary of using UV light to check antiques etc, scroll down or click here

Technical explanation of wavelengths and which are best, scroll down or click here

Guide to which products to buy, scroll down or click here

PLAIN ENGLISH SUMMARY OF USING UV LIGHT TO CHECK ANTIQUES etc

Ultra violet light (known is the U.S.A. as 'black light') is a very intense blue light, it is not visible to the human eye. Fluorescent-tube UV torches give off a very dull purple or violet glow. L.E.D. UV torches give off a dull lilac-blue. But the actual UV light is invisible, so the only way you can tell how effective it is - is to observe its curious effects.

If porcelain has been repaired, the missing part will have been 'rebuilt' or the crack glued, and then carefully repainted so that the colour matches. But the colour will only match perfectly in white light (e.g. daylight), under UV light the new paint shows up a totally different shade. This applies to many decorative items, watch dials, dolls heads, even faint signatures on letters and modern touch-up paint on old paintings.

Other examples in the world of antiques and art: oil paintings, art on paper, marble, jade and ivory, all show characteristic patterns under UV light:

How does it work? It works by reflecting off the surface and showing up the difference in the chemical composition of the paint, or by showing characteristics of the item's basic chemical make-up. You have probably noticed the same effect when looking at a car parked under a yellow street light, new paint on the car can show up quite a different shade, whereas in daylight it matches perfectly.

There are other examples in every day life.

There is starch in ordinary paper that makes it glow a vivid blue when lit with ultra violet light, but banknotes (which are made of a linen-type material and not paper) merely look 'slightly blue', hence these lights are used as 'forged banknote detectors'. Many banknotes are also printed with 'secret' marks, using UV-ink, which can only be seen under UV light.

Additives in soap powders glow under ultra violet light, they are added for that very reason: UV light from the sky makes your white washing glow blue-white, so that the advertisers can say that their powder 'washes whiter than white'.

Quinine is added to drinks (e.g. tonic water) to give a bitter taste. And quinine glows under UV light. So the next time you are in a dimly-lit bar drinking a gin and tonic, shine your UV torch on it and see it glow.

Most dramatic of all is Vaseline Glass (Uranium Glass) which is so sensitive to UV light that it glows brilliantly.

Most mammals, including man, cannot see UV light, but reindeer living in the Arctic can. To a reindeer, urine and lichens appear black against the ice and snow as they absorb UV light, lichens are good (reindeers eat them), urine is bad (indicates nearby predators). UV light is used by the forensic services, because blood and urine fluoresce, and so a powerful UV light can show traces of blood at a crime scene.

Here are four questions we get asked about UV light.

Question: so why do some UV lights appear really bright and others appear really dim?

Answer: many popular UV torches are 'popular' because they appear 'really bright' to the human eye, but the amount of actual UV light (which is invisible to the human eye) is very small, and their effect (as a UV light) is very slight...to the point of being useless. By contrast, some UV torches appear dim to the human eye, but the amount of actual UV light is large and they work well. We have tested each of our products and each one in our store includes the test results.

Question: do I have to use a UV torch in the dark?

Answer: the effect is certainly spectacular if you go into a completely dark room, but usually it is sufficient to use it in dim light. Use a square of dark cloth, or the underneath of a table in a dim corner or even the inside of your jacket (you've probably seen dealers lifting their jacket and, it would seem, examining their armpit with a blue light). UV lights used in shops for testing banknotes include a shade. This is no different from using an ordinary torch, turn it on outside in bright sunlight and you won't see any effect.

Question: can't I just use a brighter UV torch, so that I don't have to use it in 'dim' light?

Answer: To an extent - yes. But it depends on the type (wavelength) of the light, if it is wrong (as with all the cheap UV torches) then it doesn't matter how bright the light, it simply won't work. Providing the type (wavelength) is right, then yes, a larger brighter UV torch will light up a wider area and work in brighter light, just the same as an ordinary torch. Scroll down or click here for recommendations.

Question: if you use it by shining the light on the surface of an antique, does that mean that it won't see below the surface?

Answer: Exactly so! If the item has been repaired and then re-glazed, a UV torch will be of no use, there will be no 'new' paint to see, the entire surface will be new. Similarly, you cannot see through layers of paint to see 'hidden paintings', UV light is not the same as x-rays. But you can see differences in surface paint, and that can indicate that a painting has been 'touched up'. Similarly, if a signature reacts differently to the surrounding work, it's clearly made of a different paint and may have been added.

TECHNICAL EXPLANATION OF WAVELENGTHS AND WHICH ARE BEST

The electromagnetic spectrum ranges from AM at one end (which includes Medium Wave and Long Wave radio) to Gamma Rays at the other. The measurements on the right of the chart, below, are the length of each wave section (from peak to trough) in namometres (nm). 1nm = 0.000001mm.

As you see from this chart, "visible light" falls between microwaves and x-rays. Although close to x-rays, UV light can't penetrate the human body, its effect (for shortwave UV) is limited to burning the surface of skin (and eyes). Having your skin burnt by shortwave UV light is very popular, it's called 'getting a sun tan', and if there is no sun, you can get yourself grilled, under giant UV lights, on a 'sun bed'.

The tiny band we call 'light' ranges from infrared (the human eye can't detect far-infrared) to ultraviolet (the human eye can't detect ultraviolet):

And within this, the tiny bit we call 'ultraviolet light' looks like this:

365nm - 302mn - 254nm

UV light is divided into three types depending on the wavelength (these classifications are approximate, since there's nothing to distinguish the boundary from one to another):

"Longwave" UV, also known as UVA, 400 to 300nm

"Midrange" UV, also known as UVB, 315 to 280nm

"Shortwave" UV, also known as UVC, 280 to 250nm

All UV light is invisible to the human eye, the UV torches you buy also give out some visible light and it is this you can see as a dull glow. Manufacturers of the cheaper torches ensure that this visible glow is a bright, so that the user says, "Wow, this UV torch is so powerful!". Giving out some visible light is, of course, useful, otherwise you wouldn't know if your UV torch was switched on or off.

All the UV lights we sell are longwave (shortwave not only burn skin, but can damage eyes, they are usually encased in 'viewing cabinets'), and this is how the wavelengths compare, my standard test being a British twenty pound note:

395nm: the short lines are clearly visible at once but the "20" is not visible, not even if you use the it in the dark, e.g. Premierlight PL-UV, £37.50

385nm: the short lines are visible; the "20" is just about visible, but only in the dark, e.g. model: UV-KR-2 £3.90.

365-370nm: both the short lines and the "20" are clearly visible. Model: all our other models.

RECOMMENDATIONS

This is how the wavelengths compare, my standard test being a British twenty pound note, on the side with the portrait of the Queen, there are a myriad of fine lines on the left, and a big number '20' in the very middle at the top:

395nm: the short lines are clearly visible at once (the six L.E.Ds cover a wide area), but the "20" is not visible, not even if you use the it in the dark. Premierlight PL-UV, £37.50. This proves my point that making the level of light really bright does not help if the wavelength is wrong. On the plus side: this is a well-known branded UV torch (which also includes an ordinary, white light, torch) at a really good price...for this brand.

385nm: the short lines are visible; the "20" is just about visible, but only in the dark. UV-KR-2 £3.90. The worst of all worlds, a wavelength that is too long to work well, and a tiny light (one L.E.D.) that will light up just a few milimetres area at a time. So why should you buy it? Well, it works very slightly and it doesn't cost much.

365-370nm: both the short lines and the "20" are clearly visible. These work well, the various models are of two types, the old-fashioned fluorescent tube has the advantage that they really don't cost much and are of a reasonable brightness: there's a battery-operated version, UV £12.50, or a mains-powered version for testing banknotes, uv-note-checker, £15.00. The other models all have L.E.D. lights, the difference is in the number of L.E.D. lights each torch has: UV-KR £8.50 works well but covers only a tiny area (a few millimetres); UV-12 £27.50 is my favourite, because it's a neat torch, and it has 12 L.E.D. lights which covers a good area (a few inches); UV-68 £120.00 has 68 L.E.D. lights and the UV is so bright that it lights up a huge area (a few feet) and works in ordinary room light.